Thanks @smb127! One method I use is comparing a picture of a real car from google for example plus my picture of X car and analyze them closely looking at different areas to see if they all match.

@King_AM
There are several indicators as to if it is a real RS or not, but at the same time you must be careful because there are really convincing tribute cars out there. I have no doubt that this is indeed a real RS, and to determine that the spotter may have considered a number of clues such as; the black horn-grilles, front-mounted oil cooler hole in bumper, no side deco trim, the square mirror, and the white lens euro driving light under the headlight. It is tricky though because the RS (much like todays Porsches) were able to be optioned very highly giving a possibility of several different styles of markings and features that not everyone would expect a RS to have. This was a great find. I hope my explanation helped.

Awesome spot! How can you tell a real RS from a fake? I've seen a blue Carrera with that same decal on the side but had no clue if it's a legit one.